In order to measure surface features and the longitudinal profile of a road to be inspected, a number of pavement condition indicators and characteristics are measured. The International Roughness Index (IRI) characterizes the pavement condition. FIG. 1 (Prior Art) shows the longitudinal profile measurement in the ideal case. An aerial view of a section of a road to be inspected 100 is shown. The inspection vehicle 102 travels between the lane markings 104 along a trajectory 106 following the longitudinal axis 108 of the road. The number of measured profiles in the transversal axis 110 captured by the inspection vehicle 102 with its inspection system 112 can extend from 1 to N where N is the number of 3D measurement points available in the transversal axis 110. In most systems, the number of 3D points is limited to one or two. In the special case where N is high and the 3D measurement points are distributed so as to cover the width of the road, an image representing the road elevation can be formed. The inspection vehicle 102 travels between the lane markings 104 and captures the longitudinal profile measurement 116 at a predefined distance d to one of the lane markings 104. In the example shown, the predefined distance d is from the right hand side marking 118.
In conventional systems, an acquisition instrument called a profilometer is used for the measurement of the longitudinal profile of roads. This acquisition instrument includes two single point range sensors and Inertial Measurement Units (IMUs) mounted in the wheel path of the inspection vehicle. The single point range sensors are used to measure the distance between the IMUs and the road and the IMUs are used to estimate the total change in elevation of the road and the inspection vehicle while in motion. By subtracting the two measurements, it is possible to measure only the variations in elevation of the road surface, that is, the longitudinal profile. Most of the time, the integration of the signal from a vertically oriented accelerometer (the simplest form of IMU) can be used to track the total elevation changes of both the road surface and the inspection vehicle. FIG. 2 (Prior Art) illustrates an acquisition instrument used to measure the longitudinal profile of the road 100. The acquisition instrument 200 is equipped with two single point 3D sensors 202 and two IMUs 204 or two accelerometers mounted on the inspection vehicle 102. The 3D sensors 202 are usually positioned in the wheel path.
In some systems, the single point range sensors are replaced with multipoint laser line profilers that cover a road width of a few inches. These types of laser line profilers are used to compensate for different road surface textures such as longitudinally tinned (striated) concrete surfaces. FIG. 3 (Prior Art) shows an alternative configuration 300 for the longitudinal profile measurement using a line of single point 3D sensors 302. IMUs 304 can still be present. This configuration adds robustness against texture variations on the road surface.
Both the profilers using single point range sensors and the limited width laser line sensors are very sensitive to the lateral shift of the inspection vehicle.
To help the driver follow the same trajectory in each run, a guide line is often painted on the road surface. Even guided as such, it is very difficult for the driver/operator to perfectly align the profiler with the reference line and to do this with little positional variations for multiple passes while driving at highway speeds. Lateral movement will occur. Since the measurement trajectory is different for each survey even when captured on the same road section, the longitudinal profile and the indicators calculated will also be different. FIG. 4 (Prior Art) shows the longitudinal profile measurement 400 with a single point system when lateral movement is present. Since the inspection vehicle followed a wavy trajectory 402 on the road 100, the resulting longitudinal profile 404 was also measured along a wavy trajectory instead of being measured on the ideal straight line trajectory 116.
This non-ideal measurement trajectory 402 limits the performance and repeatability of the system for road monitoring applications.